Orthogonal frequency-division multiplexing (OFDM) signaling schemes dominate the existing and future wireless communication systems due to the robustness of OFDM against multipath delay spread and the flexibility of OFDM in spectrum usage, making OFDM a strong candidate for dynamic spectrum access (DSA)-based networks.
Despite the advantages of OFDM, an OFDM signal exhibits high spectral sidelobes as a result of the rectangular transmit pulse. An Orthogonal Frequency Division Multiplexing (OFDM) signal consists of sinusoidal signals that are shaped with the rectangular function to generate the rectangular transmit pulse. As a result of the shaping of the sinusoidal signals utilizing a rectangular function, the OFDM signal suffers from high out-of-band radiation due to high spectral sidelobes of sine shaped subcarriers, resulting in adjacent channel interference (ACI). Additionally, the rectangular windowing used at the receiver results in a frequency response of the receiver filter (i.e. sine) having weak ACI rejection capability.
Two common countermeasures are known to reduce the out-of-band (OOB) radiation, the first is known as time domain windowing and the second is known as frequency domain guard insertion. In time domain windowing, the rectangular pulse is filtered by a windowing function that smooths the symbol transitions. In frequency domain guard insertion, edge-band subcarriers are nulled to reduce spectral leakage to the out-of-band.
Additionally, there exist active techniques to suppress the spectral sidelobes. While these techniques give good suppression results, they are primarily computationally complex due to the data-dependent optimization problem and symbol-by-symbol processing. In addition, some of the techniques known in the art increase the peak-to-average power ratio (PAPR), degrade the bit error rate performance and require data dependent information for symbol recovery.
Alternatively, time domain windowing techniques with guard subcarrier insertion are a much simpler and computationally efficient alternative to reduce out-of-band radiation for OFDM based systems. In addition, the guarding effect of time domain windowing against multipath delay spread is an inherent advantage of the windowing technique over the aforementioned scheme.
In accordance with the Uncertainty Principle, a signal cannot be simultaneously limited in time and frequency. As such, a rectangular transmit pulse trades the spectral confinement in OFDM. With windowing techniques, containment in the time domain is relaxed to achieve a better spectrally-localized signal. However, conventional windowing schemes, e.g. commonly used raised cosine (RC) windowing, do not provide the optimum solution for time-frequency localization. In other words, the available frequency band near the main band of the signal, which can be the guard band between the adjacent channels or the band between the edge subcarrier and the spectral mask to be complied, is not maximally utilized with the conventional windowing techniques.
While other windowing functions may be utilized to reduce the out-of-band (OOB) power, these windowing functions do not optimally address the need for the reduction and rejection of ACI.
Accordingly, what is needed in the art is an OFDM windowing technique that provides ACI suppression and rejection.